Power supply switch circuit with current leakage protection

ABSTRACT

A power supply switch circuit with current leakage protection is used in an electronic device, and is able to prevent the erroneous actions caused by a leakage current in high-temperature environments. The power supply switch circuit comprises a transistor having an emitter, a base and a collector, a field effect transistor having a drain, a source and a gate, and at least two series-connected diodes. The emitter is connected to a voltage source. The collector is connected to a load via a forward biased diode. The drain receives a trigger signal. The source is connected a reference terminal via at least two series-connected diodes. The gate is connected to the collector via a second resistor. One terminal of a third resistor is connected to the gate, and the other terminal is connected to the reference terminal.

RELATED APPLICATIONS

This application is a Divisional patent application of application Ser.No. 11/330,158, filed on 12 Jan. 2006 now abandoned. The entiredisclosure of the prior application, Ser. No. 11/330,158, from which anoath or declaration is supplied, is considered a part of the disclosureof the accompanying Divisional/Continuation application and is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply switch circuit withcurrent leakage protection and, more particularly, to a circuit used inan electronic device and capable of preventing the erroneous actionscaused by a circuit leakage current in high temperature environments.

2. Description of Related Art

As shown in FIG. 1, a prior art power supply switch circuit is primarilyformed by connecting two transistors Q₁, Q₂ and other electroniccomponents. When power is to be supplied, a trigger signal S_(T) is usedto control the transistor Q₁ to be on so that a voltage source V_(CC)can provide power to a load 90 via the transistor Q₁. Meanwhile, thesupplied power can also be sent to the base of the transistor Q₂ to makethe transistor Q₂ on. After the transistor Q2 is on, the base of thetransistor Q₁ will be connected to a reference terminal Gnd to keep thetransistor Q₁ on. When the power is to be cut off, a controller 91 willsend a cut-off signal to the transistor Q₂ to make the transistor Q₂off. The transistor Q₁ will then be off in succession so that thevoltage source V_(CC) can no longer provide power to the load 90.

The leakage current of a transistor depends on the working temperature.The magnitude of the leakage current is proportional to temperature.Please refer to FIG. 1 again. According to the transistor'scharacteristics, when the circuit stops providing power, the transistorQ₂ will have a leakage current I_(CEO). When the temperature rises, theleakage current I_(CEO) increases, and the base current I_(B1) flowingthrough the transistor Q₁ increases therewith. Because the collectorcurrent I_(C) is β times the base current I_(B) according to thetransistor's characteristics, the collector current I_(C1) of thetransistor Q₁ will increase with the base current I_(B1) of thetransistor Q₁. Similarly, the base current I_(B2) of the transistor Q₂increases with the collector current I_(C1) of the transistor Q₁. Withthe increase of the base current I_(B2) of the transistor Q₂, theleakage current I_(CEO) increases. In this vicious circle, the powersupply switch circuit will generate erroneous actions. That is, when thepower supply switch circuit is originally in the off state (i.e., stopsproviding power), the transistor Q₁ will be on due to the leakagecurrent I_(CEO) to drive the power supply switch circuit into the onstate (i.e., starts providing power).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a power supply switchcircuit with current leakage protection, which is used in an electronicdevice and capable of preventing the erroneous actions caused by acircuit leakage current in high temperature environments.

The present invention comprises a transistor, a field effect transistorand a third resistor. The transistor has an emitter, a base and acollector. The emitter is connected to a voltage source. The collectoris connected to a load via a forward biased diode. The field effecttransistor has a drain, a source and a gate. The drain receives atrigger signal. The source is connected to a reference terminal via atleast two series-connected diodes. The gate is connected to thecollector via a second resistor. One terminal of the third resistor isconnected to the gate, and the other terminal is connected to thereference terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects and advantages of the present invention will be morereadily understood from the following detailed description when read inconjunction with the appended drawing, in which:

FIG. 1 is a circuit diagram of a prior art power supply switch circuit;

FIG. 2 is a circuit diagram of a power supply switch circuit withcurrent leakage protection of the present invention; and

FIG. 3 is a circuit diagram showing how the present invention is used inan electronic device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a circuit diagram of a power supply switch circuit withcurrent leakage protection of the present invention. The power supplyswitch circuit with current leakage protection is used in an electronicdevice (not shown), and can prevent the erroneous actions caused by aleakage current in high temperature environments. The power supplyswitch circuit with current leakage protection comprises a transistorQ₃, a field effect transistor Q₄ and a third resistor R₁₄. Thetransistor Q₃ has an emitter E, a base B and a collector C. The emitterE is connected to a voltage source V_(CC). The collector C is connectedto a load 92 via a forward biased diode D₁₃. The field effect transistorQ₄ has a drain D, a source S and a gate G. The drain D is connected tothe base B of the transistor Q₃ via a first resistor R₁₂, and receives atrigger signal S_(T1). The source S is connected to a reference terminalGnd via a voltage clamp unit 93. The voltage clamp unit 93 provides areference voltage, and is formed by series connecting at least twodiodes (D₁₁, D₁₂) together in this embodiment. The gate is connected tothe collector C of the transistor Q₃ via a second resistor R₁₃. Oneterminal of the third resistor R₁₄ is connected to the gate G of thefield effect transistor Q₄, and the other terminal is connected to thereference terminal Gnd.

The second resistor R₁₃ and the third resistor R₁₄ are series connectedtogether to form a voltage divider unit 94. The power supply switchcircuit with current leakage protection of the present invention furthercomprises a capacitor C₁₁ and a fourth resistor R₁₁, which are parallelconnected between the emitter E and the base B of the transistor Q₃. Thetransistor Q₃ is a PNP transistor, and the field effect transistor Q₄ isan N-channel field effect transistor.

Please refer to FIG. 2 again. When the power supply switch circuitsupplies power normally, it receives a low-level trigger signal S_(T1)for the activation of power supply. The low-level trigger signal S_(T1)drives the transistor Q₃ to be on. After the transistor Q₃ is on, thevoltage source V_(CC) provides power to the load 92 via the transistorQ₃. Because the transistor Q₃ is on, the voltage source V_(CC) willproduce a large enough control bias on the resistor R₁₄ of the voltagedivider unit 94 to drive the field effect transistor Q₄ to be on. Whenthe control bias is larger than the sum of the on voltage of the fieldeffect transistor Q₄ and the reference voltage, the field effecttransistor Q₄ immediately enters the on state. After the field effecttransistor Q₄ is on, the base B of the transistor Q₃ is connected to thereference terminal Gnd to keep the transistor Q₃ on, thereby continuingproviding power to the load 92.

When the circuit stops providing power, the field effect transistor Q₄will generates a leakage current I_(DSS). When the temperature rises,the leakage current I_(DSS) increases, and the base current I_(B3)flowing through the transistor Q₃ increases therewith. Because thecollector current I_(C) is β times the base current I_(B) according tothe transistor's characteristics, the collector current I_(C3) of thetransistor Q₃ will increase with the base current I_(B3). The increaseof the collector current I_(C3) of the transistor Q₃ will cause anincrease in the voltage of the gate G of the field effect transistor Q₄.However, because the source S of the field effect transistor Q₄ isseries connected to the voltage clamp unit 93 (i.e., at least twoseries-connected diodes D₁₁ and D₁₂), for the field effect transistor Q₄to be on, the voltage of the gate G has to be larger than the sum of thepinch-off voltage V_(T) of the field effect transistor Q₄ and thevoltage drop across the at least two series-connected diodes D₁₁ andD₁₂.

When the present invention works in high-temperature environments, theincrease of the leakage current I_(DSS) will lead the base currentI_(B3) and the collector current I_(C3) of the transistor Q₃ to rise insuccession. However, because the voltage of the gate of the field effecttransistor Q₄ generated when the collector current I_(C3) flowingthrough the third resistor R₁₄ is still smaller than the sum of thepinch-off voltage V_(T) of the field effect transistor Q₄ and thevoltage drop across the at least two series-connected diodes D₁₁ andD₁₂, the field effect transistor Q₄ won't cause erroneous actions evenif there is any variation in temperature. Therefore, the power supplyswitch circuit can work stably without any influence from transistor'sleakage current in high-temperature environments.

FIG. 3 is a circuit diagram showing how the present invention is used inan electronic device. An electronic device 10 receives a high-leveltrigger signal S_(TR) for the activation of power supply. The high-leveltrigger signal S_(TR) is connected to a power supply switch circuit 20via a first switch unit 30, and is connected to a microprocessor 50 viaa second switch unit 40. The high-level trigger signal S_(TR) drives atransistor Q₅ in the first switch unit 30 to be on. After the transistorQ₅ is on, the base of the transistor Q₃ is connected to the referenceterminal Gnd so that the transistor Q₃ is on. The voltage source V_(CC)thus provides power to a load 60 and a voltage regulator 70 via thetransistor Q₃. Because the transistor Q₃ is on, the voltage sourceV_(CC) will produce a large enough control bias on the resistor R₁₄ ofthe voltage divider unit 94 to drive the field effect transistor Q₄ tobe on. When the control bias is larger than the sum of the on voltage ofthe field effect transistor Q₄ and the reference voltage, the fieldeffect transistor Q₄ immediately enters the on state to keep thetransistor Q₃ on.

When the electronic device 10 is to stop providing power, it provides alow-level trigger signal S_(TR) for end of power supply. The low-leveltrigger signal S_(TR) drives a transistor Q₇ in the second switch unit40 to be off. After the transistor Q₇ is off, the microprocessor 50immediately outputs a high-level signal to drive a transistor Q₆ in athird switch unit 80 to be on. After the transistor Q₆ is on, the gate Gof the field effect transistor Q₄ is connected to the reference terminalGnd to drive the field effect transistor Q₄ to be off. After the fieldeffect transistor Q₄ is off, the transistor Q₃ immediately enters theoff state, and the voltage source V_(CC) stops providing power to theload 60 and the voltage regulator 70.

When the electronic device 10 stops providing power, if the temperaturerises, the leakage current I_(DSS) of the field effect transistor Q₄will increase, and the base current I_(B3) flowing through thetransistor Q₃ increases therewith. Because the collector current I_(C)is β times the base current I_(B) according to the transistor'scharacteristics, the collector current I_(C3) of the transistor Q₃ willincrease with the base current I_(B3). The increase of the collectorcurrent I_(C3) of the transistor Q₃ will cause an increase in thevoltage of the gate G of the field effect transistor Q₄. However,because the source S of the field effect transistor Q₄ is seriesconnected to the at least two series-connected diodes D₁₁ and D₁₂, forthe field effect transistor Q₄ to be on, the voltage of the gate G hasto be larger than the sum of the pinch-off voltage V_(T) of the fieldeffect transistor Q₄ and the voltage drop across the at least twoseries-connected diodes D₁₁ and D₁₂.

When the present invention works in high-temperature environments, theincrease of the leakage current I_(DSS) will lead the base currentI_(B3) and the collector current I_(C3) of the transistor Q₃ to rise insuccession. However, because the voltage of the gate of the field effecttransistor Q₄ generated when the collector current I_(C3) flowingthrough the third resistor R₁₄ is still smaller than the sum of thepinch-off voltage V_(T) of the field effect transistor Q₄ and thevoltage drop across the at least two series-connected diodes D₁₁ andD₁₂, the field effect transistor Q₄ won't cause erroneous actions evenif there is any variation in temperature. Therefore, the power supplyswitch circuit can work stably with out any influence from transistor'sleakage current in high-temperature environments.

To sum up, the present invention makes use of a PNP transistor and anN-channel field effect transistor to achieve current leakage protectionso that the circuit won't produce erroneous actions due to the increaseof leakage current in high-temperature environments.

Although the present invention has been described with reference to thepreferred embodiment thereof, it will be understood that the inventionis not limited to the details thereof. Various substitutions andmodifications have been suggested in the foregoing description, andother will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

1. An electronic device receives a start signal for the activation ofpower supply, comprising: a first switch unit receiving the start signaland outputting a trigger signal; a PNP transistor connected to the firstswitch unit and conducted by the trigger signal; a voltage divider unitconnected to said PNP transistor and used to output a control biasaccording to a conduction current of said PNP transistor; an N-channelfield effect transistor connected to said PNP transistor and saidvoltage divider unit, conduction of said N-channel field effecttransistor being controlled by said control bias and used to maintainsaid PNP transistor in an on state; and a voltage clamp unit connectedto said N-channel field effect transistor and providing a referencevoltage to increase the level of an ON voltage of said N-channel fieldeffect transistor; a microprocessor; and a second switch unit connectedto the microprocessor, wherein the second switch receives the startsignal and controls the microprocessor to turn on the N-channel fieldeffect transistor via a third switch unit.
 2. The electronic device asclaimed in claim 1, wherein said N-channel field effect transistor isconnected to said PNP transistor via a first resistor.
 3. The electronicdevice as claimed in claim 2, wherein said voltage divider unitcomprises a second resistor and a third resistor being connected inseries relation.
 4. The electronic device as claimed in claim 3 furthercomprising a capacitor and a fourth resistor, wherein said capacitor andsaid fourth resistor are connected in parallel relation between anemitter and a base of said PNP transistor.
 5. The electronic device asclaimed in claim 4, wherein said voltage clamp unit comprises at leasttwo diodes, and said diodes are connected in series relation.
 6. Theelectronic device as claimed in claim 1, further comprising a load and avoltage regulator connected to the PNP transistor for receiving thevoltage source respectively.